Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation. They may also take the form of implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, which electrically couple the pacemaker to the heart.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
A popular mode of operation for dual-chamber pacemakers is the DDD mode. Specifically, DDD systems provide atrial pacing during atrial bradycardia, ventricle pacing in the setting of overt AV block or even compromised AV nodal conduction such as first degree AV block, and atrial and ventricular pacing during combined atrial and ventricular bradycardia or heart block also known as AV block. In addition, DDDR systems monitor patient activity levels for controlling pacing rate to more closely approximate the normal response of the heart to exercise, or other physiological activity demanding a faster heart rate.
Recent studies have indicated, however, that ventricular pacing in the setting of intact AV nodal conduction may have an adverse impact compared to permitting intrinsic ventricular contractions. Hence, pacing therapies have been advanced which encourage intrinsic ventricular activity while still providing back-up ventricular support should AV block develop. One such system employs an algorithm labeled auto intrinsic conduction search (AICS) wherein the pacemaker utilizes two AV intervals. The first interval is a programmable base AV interval to support ventricular demand pacing. It may be, for example, on the order of two hundred (200) milliseconds. The second AV interval is an extended AV interval which may be thought of as comprising the base AV interval with an AV interval extension added to its end. The AV interval extension may be on the order of one hundred (100) milliseconds, for example. Hence, in this example, the extended AV interval would be on the order of three hundred (300) milliseconds.
The AICS may be implemented as follows. Over a preset interval, for example five minutes, the device paces in a demand mode with the base AV interval. At the end of the preset or programmable time period, the algorithm extends the AV delay searching for intact AV nodal conduction. If a native QRS complex or “R wave” is detected during that extended AV delay, the extension remains in effect and the system functions as if it were a single chamber atrial pacemaker. The device does not reset to the shorter AV interval unless overt AV block occurs such that there is one cycle of AV pacing at the extended AV delay or the atrial rate exceeds a set upper atrial rate limit at which time the AV interval extension is canceled with the system returning to the programmed base AV delay. Following this, even if the rate slows below this set upper atrial rate limit, the programmed base AV delay remains in effect until the time-out occurs and a search for intrinsic conduction is again performed automatically by the algorithm. The upper atrial rate limit may be either preset on the order of ninety (90) beats per minute (bpm), for example or programmable.
Unfortunately, some patients with conduction delays at higher atrial rates run the risk of canceling the AICS feature if they have frequent premature atrial complex (PACs). A premature atrial complex is an atrial depolarization occurring early with respect to the basic sinus cycle. It is not unusual for a PAC to result in an effective atrial rate greater than the upper atrial rate limit for the atrial cycle in which it occurs. Hence, such an occurrence can run the risk of resetting the AICS extended AV interval to the base AV interval and reinitiate the five minute period of demand pacing at the shorter, base AV interval before a search is performed. This is indeed unfortunate because, PACs are generally isolated events although occasionally they can occur in short salvos. It would, of course, be desirable for patients with intact AV conduction who experience PACs to benefit from intrinsic ventricular activity and features such as AICS and not have this algorithm effectively canceled by transient non-sustained events.